Nuclear spectroscopy without compton background



SOURCE RECORDER FI G. 2 B

D. O. SEEVERS Filed Oct. '7, 1954 GATE AMP.

SHIFT Moo.

POWER PULSE lNVE NTOR DEL/WAR o. $55 VERS PHASE HOR Sept. z, 195s SUPPLY nit@ Sas 2,856,642 Patented Sept. 2,1958

NUCLEAR SPECTRUSCOPY WITHOUT COMP'IN BACKGROUND Delmar 0. Seevers, Fullerton, Calif., assignor to California Research Corporation, San Francisco, Calif., a corporation of Delaware Appiication October 7, 1954, Serial No. 460,926 7 Claims. (Cl. Z50-7l) The present invention relates to a method of and apparatus for recording a spectrum of nuclear energies, more particularly to a method for recording and transmitting a spectrum of neutron-capture gamma rays priman'ly representing the pair-production and photoelectric effect from an earth formation to a surface recording system, and has for an object the provision of a system adapted to receive selectively, but substantially simultaneously, nuclear energies, such as gamma rays arising from an earth formation during neutron irradiation, recording said energies detected by at least a pair of detectors as electrostatic charges on an intermediate storage system to integrate the relative abundance of particular gamma ray energies when interacting with said detectors either by the Compton effect or by the pairproduction and photoelectric effect, and then transmitting to the earths surface a low-frequency signal modulated in accordance with the difference in distribution of said electrostatic charges when one of said detectors is responsive to both pair-production, photoelectric and Compton effects, While the other is responsive only to Compton effect, to produce a record of said pair-production and photoelectric effects substantially free of the Compton background effect, normally present in the recording of neutron-capture gamma ray spectra.

ln the detecting and recording of energies of neutroncapture gamma rays produced by the irradiation of an earth formation along a well bore by a neutron source, it is found that the use of a single scintillation crystal and photomultiplier tube combination results in a recorded energy spectrum that is made up of a continuum with peaks superimposed upon it. Though the energies of the gamma rays produced in the earths formation are discrete and a function of the elements excited by the neutrons, the continuum results from: (l) degradation or" the energy of some of the gamma rays in the earth formation by the Compton effect before the gamma rays enter the detector, and (2) interaction by the Compton effect of the gamma rays that enter the scintillation crystal. The peaks superimposed on the continuuni are due to gamma rays interacting with the crystal by the photoelectric and pair-production effects. Since the peaks are the components of the spectrum that make it possible to identify individual constituents of an earth formation, the diagnostic characteristics of the measured signals may be greatly increased if the continuum due to the Compton effect in the crystal can be substantially reduced in the recorded signal.

Various methods have been suggested for elimination of the background of Compton effect, such as by use of a plurality of crystals connected as individual scintillation counters connected in circuits which exclude all detected nuclear events which do not occur simultaneously in the plurality of crystals. Such selective interaction and recording of only those events occurring simultaneously in a plurality of detectors, presents serious limitations as to the speed with which the logging apparatus may be drawn through the Well bore to record a statistically reliable number of nuclear interactions. However, I have found that by ythe provision of at least two crystals, one of which is primarily responsiveto the Compton effect and the other of which is responsive to the Compton, photoelectric andpair-production effects, there may be provided a logging system wherein the apparatus for irradiating and detecting may be operated at a relatively high speed, without loss of the diagnostic information normally involved in the operation of a plu. ralty of crystals connected as a pair spectrometer. Additionally, said system provides an arrangement not only for detecting both the Compton effect and the plural effects of Compton, photoelectric and pair production, but also for ,transmitting the resultant signals to the earths surface for recording during the logging mn. In accordance with a preferred form of apparatus for carrying out the present invention, said signals are stored and transmitted by a system similar to that disclosed in my copending application, Ser. No. 433,244. As therein disclosed, the individual nuclear events are recorded as electrostatic charges on a storage surface and positioned thereon in accordance with their individual energies. The accumulated electrostatic charges may then be transmitted over a conventional well logging cable of limited power and narrow frequency-band characteristics.

In accordance with an important aspect of the present invention, there is provided a system for storing the characteristic bits of information representing the interaction of individu-al gamma rays with a scintillation crystal, primarily responsive to Compton scattering, and at the same time recording similar bits of information representing the interaction of gamma rays with a scintillometer responsive not only to Compton scattering but also to photoelectric and pair-production effects. Preferably, the signals from each of the detectors is alternately accumulated in a predetermined position on ,an electrostatic storage surface of a cathode ray storage tube, with each bit of information being distributed in an area corresponding to its source and in accordance with the magnitude of each gamma ray detected by said source. In a preferred manner of operation, these bits of information are stored on a circular storage target'with the low-energy gamma rays being accumulated as electrostatic charges around the outer periphery of said storage surface and the higher energy gamma rays being deposited as electrostatic charges near the center of the storage surface. Each `of the electrical pulses of equal magnitude, representing the energy of the gamma rays, surrendered to the crystals by interaction therewith, are deposited on circles of equal radius but with the gamma rays arising from one crystal being deposited on one half of the storage surface, and those from the other crystal on. the other half. The target surface is then circularly scanned in a spiral manner and the resultant signal treated in such a way as to subtract the effect of the scintillation counter representing only Compton eect from the signal representing the composite Compton, photoelectric and pair-production effects. The resultant D. C. signal may then be transmitted over a conventional well logging cable to a surface recording system to produce a visual record at the earths surface which represents only the photoelectric and pair-production eiects, with the Compton scattering effect either substantially eliminated, or greatly reduced.

in a preferred form of apparatus for performing the method of the present invention, a single pulse amplifying and delay gate arrangement is alternately connected to the two gamrnaray energymeasuring scintillation detectors and the input of an'intermediate storage system alternately connected through said pulse amplier to the two scintillation counters. In said preferred form, switching of the` input to the pulse amplier is accomplished by Y input.

' a'reversingswitch sfo that the signal into the vamplifier is positive when the section Ofthetarget is scanned that `contains vthe stored information from one scintillation detector and is negative when the other section is scanned. By the Ychoice'of'proper time constants in the amplifier, the output ofthe amplier will correspond to the diierence between the switched'signals.V v Y [le Y f Further objects andadvantages of the present invention will become apparent from Vtlbie'following detailed description taken in conjunction with the accompanying drawings. v

In the drawings: Y Fig. 1A is a schematic representation of thelower end ofa well'logging sonde incorporating the present invention.

Fig. 1B is a schematic'representation ofthe upper end offthe'well loggingsonde shown in Fig. 1A (reduced in size) and its connection through the well logging cable to a surface recording system. y

Fig. 2A is aV schematic representation of the method of storing electrostatic charges on the surface ofthe cathode ray storage tube utilized in the `arrangement off-Fig. lA.

Y `.Fig. 2B is a schematic representation of the manner in which the cathode ray'bearnmay be made to scan the storage surface area of the cathode ray tube to produce a record substantially free of the background ComptonY eiecton a nuclear energy spectrum.A Y Y t f d inrparticular to the Referring 'nowV to the drawings, an

il 'arrangement of Fig. lA, there isillustrated a preferred i `form of well logging apparatus adapted to generateY and transmit a spectrum of nuclear energies representing oo nstituent elements inearth `formations traversed bythe `well bore. A 'logging sonde 10, adapted to be raised orY loweredon well'logging cableV lrrlwrithin well bore 12, is arranged to Vproduce an energy spectrumV of neutron'- capture gamma rays from earth formations, suchV as ormationl, adjacent the well logging sonde V10,by ircrystal and some by the pair-production effect which may Y either cause surrender of the entire energy of the gamma ray within the confinesY of the crystal or result in the escape of one or both annihilation quanta fromV the crystal. As employed in the recording of neutron-.capture gamma ray energy spectra by previously known systems, the energies of the individual gamma rays are measured by a single crystal responsive to the Compton effect, the photoelectric effect, and the pair-production effect. Said spectrum includes a very large number of 'gamma ray energies from which there must be isolated a small difference in such numbers to identify the generating sources of the neutron-capture gamma rays.

VIn accordancerwith the present invention, the auxiliary or other scintillation crystal 18 is preferablyV constructed Y of a material such as anthracene or other material ofV lower atomic number so that gamma rays intercepted and detected therein preferentiallyreact withthe crystal'ele- 't by pulse ampliiierr23'andthe associated electroni-c equipment, in response Vtoa Vfrequency-controlled switch or vibratorridentied as 30,V Switch 31 of -yibrator 30 is Y mentsV b y the Compton scattering or recoil effect. The

output of Veach of the two crystals j 17 and 18yis Vthen converted Yfrom light pulses to electrical pulses of similar magnitude to the intensity of the light produced in crystals 17 'and 18. As shown, Ythe'photomultiplier tubes 19 and 20 are opticallyucoupled to crystals '17 and 18,'re-

Vspectively. The output of both tubes 19 and 20 is then connected through a single input condenser 21 to a single pulse amplier 23, which is adapted V.to apply the pulses developed by photomultiplier tubes '19 and 20 to the arranged toV alternately connect'the rstdynode of the respective photomultipliertubes V19 and 20 to ground, thereby reducing'the output ofthe tube to Vsubstantially zero by shorting an early stage in that photomultiplier tube. Asrwill 'be explained more fully hereinafter, the

. power applied Yto coil 32 of vibrator 30 is synchronously controlled with the; sweep or deliection of the Vcathode ray beam within storage tube 25to provide Vcontrol of Vthe positioning of the electrostatic charges, which represent the energy andfrequency of occurrence of theVv Velectrical pulses corresponding Vtothe gamma rays detected by crystals 117 and 18.1

' Y A preferred method of operating Vthe cathode ray radiating said formation 15 with neutrons from a neutron v source'16.Y The resulting`neutron-capture gamma rays emitted by excited nuclei within theY formation,'upon capture of neutrons, reenter the'V bore hole 1K2 sol/that they may be detectedland their energies measured by the scintillation crystals 17V and 1S positioned within the well logging sonde 10. Y Y

In accordance'with the presentinvention, the crystals r17 and 18 are constructed of materialswwhich preferentially interact with'the neutron-capturegamma rays in diie'rent manners. For example, the crystal '17.may. be constructed of sodium iodide activated by Vthalliurm and,

. accordingly,rbe Ycapable of detecting gamma rays arising within formation`15 which are ofv Vsufficient energy to penetrate the two shields 13 and VY14 and'i'nteractwith the 'elements ofthe crystal, -and thereby surrenderltheir'energy'thereto, byforming klight Vpulses corresponding in magnitude to the surrendered energyof Ysaid gamma rays.

Y Such a crystal `willibe s'ens'itivreto gamma rays of relatively low energy, say 1 rn. e. v. to 3 rn. e. v., which'pref- Verentiallyinteract' by the Compton recoil effect, by surrendering a portion'of their full energies to the crystal Y by ionization and creation'of light pulses.V Additionally,

storage tube, wherein each of the gamma ray energies'is recorded and stored, contemplates the storage ofa com-V plete Yspectrum representing the constituent Velementsjof earth formation 15 so thattsaid spectrum may be recorded at the earths surface during the logging run- The present system has been illustrated as being Vcontrolled to `store suchspectrum by ,circularly scanning Vthe storage surface 24Y by the cathode ray beam. Y'

Y A detailed description of Vione method of circularly Y scanning such a Ycathode ray Vstorage tube, to record a tion.V vAs explained in the Clark application, the cathoderayf'beamY generated in storagetube 25 is essentially Vcrysta117 due to its constituents of high atomicY number, t Y

'surrender of the entire energy Vofthe gamma'ray to the l will Vcause some of the gamma raystoV interact with the e Y 'crystal elements vby the photoelectric eect which causes spectrum of nuclear energies, is disclosed Vin, U.. S'. appli-l cation Serial No. 451,525, filed August 23, 1954 in the name of Calvin M. Clark,'a co-ernployee, which application is assigned to the assignee o f the present invencontrolled in its scanning mot/ementA by electrostatic Vdeiiection plates-34 and 35;which are 4respectively connected to horizontal deflection modulatorV 36 -and vertical deflection modulator 37. Y Both modulators 36 and 37 are operated fromj aV common VAtC. source through a sineV wave oscillator 38, with the' yertical modulator being d1s`- l placed exactly ninety Vdegrees' with respectjtogthe ,horin zontal modulator by the' ninety-degree .phase-shift J networkto.. g y Y In thepreferred manner of'operating the `arrangement of Fig. 1A;deectioirmodulators 36 and 37 are operated Y at a relatively low frequency through sine wave oscillator 38 supplied from the surface A. C. source identified as 42 and transmitted to the logging sonde over Well logging cable 11 through line A. Simultaneously, as mentioned above, the vibrator 30 is operated through the same source of alternating current so that switch element 31 on each half cycle of A. C. connects the output of either photomultiplier tube 19 or 20 to the pulse amplifier 23, so that electrostatic charges of a determinable magnitude may be applied to one half of the electrostatic storage surface 24 during one half cycle by one of the photomultiplier tubes and to the other half of surface 24 during the other half cycle by the other tube.

' As indicated in Fig. 2A, these electrostatic charges, represented by the dots 45, are applied to storage surface 24 by gating the cathode ray beam to a conductive condition when each pulse representing the energy of a gamma ray has attained its maximum amplitude. In the arrangement of Fig. lA, this function is accomplished through delay gate 46 controlling the potential applied to grid 48 of storage tube 25. In storage tube 25, electrostatic charges are thus applied to the storage surface 24, which comprises an insulating surface and a conducting screen bonded together, by bombarding the insulating surface by the high-energy electron beam generated by the tube under proper accelerating potentials. When the electron beam is gated to a conducting condition, it strikes the normally insulating surface, and secondary electrons are emitted from the uniformly charged insulating coating to collector ring 49, thereby leaving a positive electrostatic charge, as represented by dots 45, on the exact area where the cathode ray beam has contacted the storage surface. As shown in the schematic arrangement of Fig. 2A, the gamma rays detected in crystal 17 are thus selectively applied to the upper half of storage surface 24. It will `be observed that the heavier dots shown in the upper half represent a large number of individual gamma rays whose energies correspond to the radii of the concentric circles. Said circles represent the bits of information obtained from crystal 17 due to the photoelectric effect, the pair-production effect and the Compton scattering effect occurring within that crystal. The opposite half of the storage surface, which is connected to pulse amplifier 23 during the other recording cycle, is arranged to store charges representing gamma rays interacting with crystal 18 which is predominantly the Compton effect alone. It will thus be seen that the two halves of the storage surface 24 contain a composite of information representing in the upper half the photoelectric effect, the pair-production effect, as well as the Compton scattering effect, while the lower half of the storage area contains only the Compton effect. The positions of the concentric circles in the present embodiment are predetermined by the energizes of the gamma rays causing the deposition of the electrostatic charges. In the preferred manner of distributing these various energies on the storge surface, those gamma rays of highest energy are positioned nearest the center of the target, While those of lower energy are progressively distributed along circles of increasing radii so that the lowest-energy gamma rays are stored as electrostatic charges around the outer periphery of the target.

-.fter a predetermined length of the formation has been traversed, or after a predetermined length of time for the exposure of a particular part of the formation to neutrons, the stored spectrum, representing the difference between that portion which includes both Compton, photoelectric and pair-production effects and that which contains only the Compton effect, may be transmitted to the earths surface over well logging cable 11 by changing the operating potentials of the cathode ray storage tube 25 to a reading or transmission condition. In the present embodiment, this is accomplished from the earths surface by a system which includes push-button 50, adapted to control the energization of relay 51 located in the logging sonde 10. The various contacts of relay 51 may thus connect the proper operating potentials to the various electrodes of tube 25 and, simultaneously, connect the horizontal and vertical modulators 36 and 37, through switch 53, to a Variable D. C. potential supplied by saw-tooth generator 54. Saw-tooth generator 54 is arranged to supply a linearly varying D. C. potential in synchronism with the regulated frequency, such as 60-cycle A. C., so that the cathode ray beam of storage tube 25 will be progressively varied in its radius to sweep circularly around the target surface 24 in the form of a spiral motion, as indicated by the line 47 in Fig. 2B. At the same time, the grid 48 is connected to a biasing potential through contact 56 of relay 51 so that the cathode ray beam will generate a signal at collector ring 49 representative of the distribution of electrostatic charges on storage surface 24 and simultaneously return that storage surface to substantially an equal potential throughout its surface.

ln the arrangement of Fig. 1A, the positively charged electrostatic areas 45 will cause electron flow from the collector ring it to the electrostatic storage surface and thereby generate a D. C.-modulated signal which may be transmitted to the surface for recording by way of line 57 of D. C. amplifier 58. In accordance with the present invention, the preferred method -of subtracting the signal of one segment of the storage target from the signal of the other segment is through synchronism vibrator 59 and amplifier 5S. Vibrator 59 is operated by sine wave oscillator 38 so that the polarity of the signal from'the collector 49 is switched synchronously with the traversal of the cathode ray beam across the segments of the storage surface 24. Amplifier 58 desirably has a time constant such that the signal transmitted over line C to the surface recorder 60 corresponds to the difference between the two stored signals. In this way, the input signal to amplifier 58 is the difference between the hits of information on the upper and lower halves of target 24, as those areas are identified in Fig. 2A. Accordingly, the desirable signal recorded is the difference between the total nuclear events occurring in crystals 17 and 18 over both a comparable period of time and a compara'ble interval of earth formation l5 along the well bore.

While, as mentioned above, the resultant signals remaining as the difference between the events detected by an anthracene crystal and a s-odium iodide crystal are desirably recorded as the difference between such detectors, it is, of course, possible to select crystals which are'sensitive to other nuclear events, and in different ratios relative to each other, so that the present apparatus may be operated to provide an integrated picture of the events detected in a plurality of crystals, with the output of each being recorded on a distinct area of an intermediate storage target, such as electrostatic storage surface 24. It will, of course, be apparent that the storage area 24 may be divided into other equal sectors, such as three equal area sectors or more. With each sector receiving electrostatic charges in accordance with the output of an individual crystal and photomultiplier tube combination, as is well understood in the art, the total difference in all of the electrostatic charges accumulated around each area of equal diameter on thestorage surface may either be added together or subtracted one from the other to provide an output signal representative of either a composite spectrum or a particular part of a single nuclear spectrum.

In the present embodiment, as mentioned above, there is transmitted to the surface recording unit 60 and recorded on chart paper 62 a spectrum which is substantially due to the photoelectric and pair-production effect, alone without the masking effect of a large number of Compton scattering reactions by gamma rays with the crystal material. The operation of the chart 62 is desirably'in synchronism with movement of the electron beam in stor-v age tube 25 and is controlled. -by the. synchronously'operated Amotor 63 which is likewise operated from common A.VC. source 4 2, asV are the Vvertical and horizontal deflection Y*modulators V36 and 37 and the saw-tooth generator 54. As shown, motor 63 begins its operation in response to Vstart switch 50, closingcontact 64 Ytoenergize motor 63. v At the same time, therelay '51 in'logging sonde 10 is energized through contact 65 iby a D. C. source, such as battery 66. As illustrated, switch 50 performs all of these operations through relay 68, which is held closed through its contact 69 and Whose operation may be interrupted by push-button 70.

An alternative arrangement for depositingthe electrostatic charges 4S on target 24 may be provided by switching the operating potentials of storage tube in synchronism with the operationVV of vibrator 30.` By causing theelectrostatic storage surface to be alternately made relatively positive and-negative during the deposition of charges, the individual charges on the upper half, for example, may be made positive relative to charges' of the -same magnitude on the lower half. In this'way, the readout of the stored charges may be accomplished by continuous scanning of the storage surface with the cathode ray beam, without requiring switching of the outputV to amplifier 58 through vibrator 59. Y

- It will be apparent that the detecting and transmission -of gamma ray spectra may be accomplished lby the abovedescribed system whether such gamma rays are of the i source of substantially pure. neutrons, detecting neutroncapture gamma rays originating in the nuclei of constituent elements within said formation with at least a pair of gammaV ray detectors, one of said detectors being sensitive to gamma rays of energies producing Compton effect absorptioneof the energies andthe other of said detectors Y being responsive to the Compton, photoelectric, and the Y pair production effects, alternatelygenerating electrical pulsesin response to the output of said detectors, convertingV said electrical pulses to electrostatic charges positionable'upon an electrostaticstorage surface withinsaid'well bore, recording the'pulses representing lgamma raygenergies detected by one of said detectors on a first portion of said electrostatic storage surface and positioning the electrostatic charges representative of the energies Ydetected by said other detector on another portion ofY said surface, accumulating said alternately'stored electrostatic chargers on said storage surface -over a predetermined'interval, generating a D.Y C. potential modulated in accordance Vwith'the comparable positions of said charges on said storagel surface, differentiating said D. C. potential inVV accordance with the difference in the number of charges ofeach energy stored on said surface, and recording s aid modulated D. C. signal at the earths surface, said record being substantially Vfree .of the Compton `background effect to emphasize substantially the photoelectric and pair production-effects and'rpermit identification of the type of nuclei from which said gamma'rays arise.

2.' ln a well loggingrapparatus for generating and transmitting a spectrum of nuclear energies toa surface recording system, means forirradiating an earth formationY with neutrons, at least a pair of of gamma'ray detectors,

Vincluding a scintillometer, one of said detectors being a scintillonieter responsive to Ysubstantially low-energy gammairays Vand, the other detector being a scintillometer responsive to .both high-energy gamrnaraysv andlow-.energy gamma rays, ariV intermediate storage meansfor accumulating'a' spectrum of nuclear energies within said well bore and adjacent saidnscintillometers circuitmeans for ap-V plying electrostatic pulses 'to oneportion of said interme diate Vstorage device in accordance with, gamma rays received by one of said detectors and applyingcharges rep-A resenting the other of said detectors on another portion of said storage surface, means for generating a D. C. potential modulatedrV in accordance with the dii'ference in distribution `of said electrostatic charges on said twoV por- Y tions of said storage surface, and'meansfor recording said D. C. potential at the earths surface as a measurement' of the relative quantity of characteristic nuclei genei'ating highY energy gamma rays in said formation duringY neutron irradiation.

3.`Well logging apparatus for recording a spectrum of selected nuclear energies ata surface recorder which comprises Vmeans for irradiating an earth formation traversed by a Well bore with a source of neutrons in said well bore, at least a pair of nuclear event detectors positioned Within said bore hole and shielded from said neutron source,

one of said detectors being responsive to a first type of nuclear event and the other being responsive to another type of nuclear event, means for converting each of said nuclear events to an electrostatic charge positionable on an electrostatic storageV surface in accordance with its energy and the detector in which said Vevent occurred, 'I

means for positioning each charge corresponding to one of said events within its prescribed storage area, means for generating a D. C. potential modulated in accordanceV Y vwith the distribution Vof all of said electrostatic chargesVV onsaid storage surface, and means for recording said D.V C. potential at the earths surface as a measurement ofthe relative abundance of characteristic elements in said formation producing said nuclear events upon irradia` tion by said neutron source, said recording being in accordance withY the depthof said source in the well bore. 4. Well loggingl apparatus for detecting and transmitting a spectrum of nuclear energies to a surface recording system comprising at least a pair of nuclear energy detectors, each comprising a scintillation crystal and a photo- Vmultiplier tube, one of said scintillation crystals beingprimarily responsive to low-energy nuclear events and the other crystal being responsive to both high-energy and low-energy nuclear events, means including said photo- Y' multiplier tube for converting each nuclear event detected in its associated crystal into an electrostatic charge, means for positioning each of said electrostaticY charges on ya storage surface in accordance with both the energy Vof said event and the crystal Vin which said Vevent originated, means for scanning said electrostatic storage surface to detect the distribution of a plurality of said charges, means for generating a D. C. potential in accordance with said Vcharge distribution, means for subtracting one portion of said D. C. potential from another `portion of 'said potential to produce arresultant signal representing substantially only the high-energy nuclear events Vrecorded on said storage surface, and means for recording said e resultant signal atthe earths surface in accordance with the' depth of said detectors iiiV said well bore:Y p

- '5. nThe method Vof recording -a gammaV ray `spectrum' generated in an earth formation traversed by a well boreV while a detector system is passing through said-bore hole, which comprise'sdetecting gamma rays originating in constituent elements within said formation by subjecting at least a pair of scintillation crystals to said gamma rays, one of said crystals being sensitive primarily to gamma rays interacting with said crystal bythe Compton scattering effect and the other of said crystals interacting Y with said gamma rays by Compton scattering, photoelec- Y Vtric wand pair-production effects, converting each interactionY of a gamma ray with one of saidY crystals to an individual-electrical pulse corresponding in magnitudeto the energy-surrendered by the gamma ray upon inter-v action with said crystal, alternately converting electrical pulses produced by gamma rays interacting with one of said pair of crystals to electrostatic charges, depositing said electrostatic charges generated by each of said pair of crystals on different predetermined portions of a storage surface positioned in the vicinity of said crystals, each of said electrostatic charges being further positioned in accordance with the energy of the interaction of said gamma rays with the crystal creating said charge, accumulating the alternately deposited electrostatic charges over a predeterminable time interval, generating a substantially D. C. potential in accordance with the comparable positions of charges on said storage surface, modulating said D. C. signal in accordance with the difference in the number of charges representing each gamma ray energy stored in response to one of said crystals and those of the same energy stored by the other of said crystals, and recording said modulated signal at the earths surface to provide a record of the gamma ray energy spectrum containing substantially only the photoelectric and pair-production eiects to permit identication of the nuclei in which said gamma rays originated in said earth formation.

6. Well logging apparatus for detecting and transmitting a spectrum of gamma ray energies to a surface recording system comprising at least a pair of scintillation crystals adapted to interact With gamma rays to produce a total number of light quanta proportional to the energy each gamma' ray surrenders in one of said crystals, one of said crystals being primarily adapted to interact with gamma rays by the Compton scattering effect and another of said crystals being primarily responsive to a combination of interactions With gamma rays including Compton scattering, photoelectric and pair-production effects, means for converting said total number of light photons to an electrical pulse of corresponding magnitude, said converting means including means for generating an electrostatic charge positionable on an electrostatic storage surface in accordance with the magnitude of said pulse, means for positioning said electrostatic charges generated by interactions in one of said crystals on a tirst predetermined portion of said electrostatic storage surface and those generated in the other crystal on another predetermined portion of said storage surface, means for scanning said storage surface to subtract the number of charges on one of said portions from those on the other portion of said surface, means for producing an electrical signal representing the ditference in said number of charges as a measure of the peaks in the gamma ray energy spectrum due to pair-production and photoelectric eects, and means for transmitting a substantially D. C. signal modulated in accordance With said electrical signal to recording means at the earths surface for visual observance of the spectrum of gamma ray energies, said recording means being operable in accordance with the depth of said apparatus in a Well bore.

7. Apparatus for detecting and transmitting a spectrum of gamma ray energies to a recording system comprising at least a pair of scintillation crystals adapted to interact with gamma rays to produce a total number of light quanta proportional to the energy each gamma ray surrenders in one of said crystals, one of said crystals being primarily adapted to interact with gamma rays by the Compton scattering eifect and another of said crystals being primarily responsive to a combination of interactions With gamma rays including Compton scattering, photoelectric and pair-production eiects, means for converting said total number of light photons to an electrical pulse of corresponding magnitude, said converting means including means for generating an electrostatic charge positionable on an electrostatic storage surface in accordance with the magnitude of said pulse, means for positioning said electrostatic charges generated by interactions in one of said crystals on a irst predetermined portion of said electrostatic storage surface and those generated in the other crystal on another predetermined portion of said storage surface, means for scanning said surface to subtract the number of charges on one of said portions from those on the other portion of said surface, means for producing an electrical signal representing the dierence in said number of charges as a measure of the peaks in the gamma ray energy spectrum due to pair-production and photoelectric eifects, means for generating a substantially D. C. signal modulated in accordance with said electrical signal, and recording means for indicating the peaks of said spectrum of gamma ray energies.

References Cited in the le of this patent UNITED STATES PATENTS 2,535,817 Skellett Dec. 26, 1950 2,659,011 Youmans et al Nov. 10, 1953 2,686,268 Martin et al Aug. 10, 1954 OTHER REFERENCES Two Crystal Gamma-Ray Scintillation Spectrometer, by R. E. Connally, from the Review of Scientific instruments, vol. 24, No. 6, June 1953, pgs. 458, 459. 

6. WELL LOGGING APPARATUS FOR DECTING TRANSMITTING A SPECTUM OF GAMMA RAY ENERGIES TO A SURFACE RECORDING STYSTEM COMPRISING AT LEAST A PAIR OF SCINITILLATION CRYSTALS ADAPTED TO INTERACT WTH GRAMMA RAYS TO PRODUCE A TOTAL NUMBER OF LIGHT QUANTA PROPORTIONAL TO THE ENERGY EACH GAMMA RAY SURRENDERS IN ONE O F SAID CRYSTALS, ONE OF SAID CRYSTALS BEING PRIMARILY ADAPTED TO INTERACT WITH GAMMA RAYS BY THE COMPTON SCATTERING EFFECT AND ANOTHER OF SAID CRYSTALS BENG PRIMARLY RESPONSIVE TO A COMBINATION OF INTERACTIONS WITH GRAMMA RAYS INCLUDING COMPTON SCATTERING, PHOTOELECTRIC AND PAIR-PRODUCTION EFFECTS, MEANS FOR CONVERTING SAID TOTAL NUMBER OF LIGHT PHOTONS TO AN ELECTRCAL PULSE OF CORRESPONDING MAGNITUDE, SAID CONVERTING MEANS INCLUDING MEANS FOR GENERATING AN ELECTROSTATIC CHARGE POSITIONABLE ON AN ELECTROSTATIC STORAGE SURFACE IN ACCORDANCE WITH THE MAGNITUDE OF SAID PULSE, MEANS FOR POSITIONING SAID ELECTROSTATIC CHARGE GENERATED BY INTERACTION IN ONE OF SAID CRYSTALS ON A FIRST PREDER- 